1. Field Of The Invention
This invention relates to flow control valves, and in particular, to an improved flow control valve for use in a liquid distribution system, such as a carbonated liquid dispensing system, the flow control valve of the present invention being useful for maintaining a constant liquid flow rate, with minimal variation from the selected flow rate, over a wide range of delivery pressures of the liquid.
2. Description Of The Prior Art
A pressure balanced flow control valve normally incorporates a spring, a piston, a sleeve, and an adjustment stem. In the conventional pressure balanced flow control valve there is a piston, which operates within a sleeve, wherein the piston is spring loaded, so that upon introduction of fluid under pressure into the chamber, the force of the fluid will act upon the piston in a direction opposite the force of the spring, the sleeve being provided with a plurality of passageways or a variable sized channel, such that movement of the piston within the sleeve will vary the size of the passageways through which the fluid is permitted to pass into a fluid outlet. The force of the spring is adjustable through the use of an adjustment member which acts upon the spring to impart greater or less force to the piston to achieve a desired flow rate.
In an alternative system, there is a piston which is connected to an adjustment stem, and moves in a reciprocal manner with respect to a sleeve, which defines a passageway, the piston on the action of a spring connected to the adjustment member, being operative to vary the size of passageway defined by the sleeve such that the flow rate of liquid through the flow control valve is controlled by the amount of spring force applied by the spring and the adjustment member balanced against the pressure of the liquid being introduced into the valve.
In the conventional pressure balanced flow control valve, a very close tolerance must exist between the piston exterior wall and the sleeve interior wall in order to control the flow of low viscus fluids, such as carbonated water. In the absence of this very close tolerance, there is leakage and an associated pressure drop, which affects the rate of liquid flow. Normally, the tolerance between the outside wall of the piston and the inside wall of the sleeve is in the range of from 0.0005 to 0.0010 inches per side. The use of this range of tolerances reduces leakage and pressure drop to a tolerable level, and maintains a desired liquid flow rate over a range of liquid delivery pressures.
However, to maintain the liquid flow rate constant, with minimal variation from the selected flow rate, for an acceptable period of time without undue maintenance or adjustment, the mating surface finish of both the sleeve and the piston must be highly polished. If the tolerance between the mating surfaces of the piston and the sleeve is not closely held, or the mating surfaces are not highly polished, there will be undesirable "blow by" at the higher delivery pressures, resulting in a variation in the liquid flow rate.
Along with the aforementioned problem of close centerline tolerance there must also be a closely held tolerance on the taper of any of the mating surfaces. Any excessive amount of taper that appears between the mating surfaces will also affect the constancy of the liquid flow rate over the normal range of delivery pressures.
The parts for conventional pressure balanced flow control valves are normally made of stainless steel or ceramic in order to obtain a very highly polished surface finish, which aids in the sliding movement of the piston within a sleeve and also reduces the amount of wear. The parts are also generally made of stainless steel or ceramic to prevent any abnormal corrosion or build up of foreign materials, which would affect the movement of the piston within the sleeve.
In the conventional flow control valve, stainless steel is generally of the 304 stainless steel type, which is non-corrosive and has a hard surface and which may be highly polished to a smooth finish. Alternately, a number of known ceramics, which also have an extremely hard surface may be polished to a very smooth finish and thus usable in this application. However, the problem with the use of ceramic is that it is brittle and thus easily breakable during manufacture, normal use or maintenance.
It is essential that the choice of either of these materials be based upon the cost, the desired durability and life of the mating parts, since wear will cause a variation in tolerance, thus affecting the constancy of the flow rate. Although stainless steel parts are generally less expensive than corresponding ceramic parts, the ceramic parts are harder and can function for a longer period of time, due to their resistance to wear. However, it is known that ceramic material is much more difficult to machine or mold and is more easily damaged.
Since the piston moves within the sleeve each time the flow of liquid is started and then stopped, which normally occurs when opening a faucet and then closing it, normal wear will occur due to the movement of the piston exterior wall riding against the interior wall of the sleeve. Generally, in soft drink dispensing applications, the continuous movement of the piston within the sleeve is such that the flow control valve needs readjustment after two or three months of use and replacement within one or two years.
Also, because of the very close tolerances that must exist between the piston and the sleeve, the fluid delivery system must be totally free of any small particles that are foreign to the fluid as these can cause the piston and sleeve to bind. For example, it is quite common for the piston and sleeve to bind as a result of mineral deposits in tap water, if the piston and sleeve in the flow control valve are stored and/or allowed to dry. If this binding occurs, a sizable force is needed to dislodge the piston and the sleeve from one another.
Also, because of these close tolerances, great care must be taken when the piston and sleeve are disassembled for cleaning and then reassembled, so as to avoid any scratch marks when one part is inserted into the other, as the slightest scratch or deformation of either of the mating surfaces of the parts will again cause the two parts to bind and become inoperative or facilitate undesirable "blow-by". If scratches or deformation occurs the parts are generally discarded and must be replaced with new parts. When these parts are made in the factory, they are generally matched in order to provide that the proper tolerances are maintained. Thus, the sleeve and piston are normally sold as a set and the destruction or damaging of any of the parts normally requires the replacement of the entire sleeve and piston assembly.
Various types of flow control valves are presently being used and have been used in the past. For example, U.S. Pat. No. 2,917,075 to C. M. Terry, entitled Flow Control Valve, illustrates a flow control valve which is used for maintaining a predetermined rate of liquid flow with only a slight variation from the desired flow rate. The flow control valve of the '075 patent includes a hollow valve member, slidably mounted in a chamber and having a conical valve seat, which is movable toward and away from an opposing valve seat to control the flow rate through the flow control valve. When the inlet pressure increases to a predetermined level over the spring pressure imparted to the spring mount in the interior of the valve, the inlet pressure causes the hollow valve member to assume a closed position, thereby controlling the outlet rate of flow. In order to increase accuracy of the valve, the valve seating member is shiftably mounted whereby it may be shifted into alignment with the conical valve seat. The compression of the valve spring is set by an adjusting screw to provide the desired pressure differential for maintenance of the desired flow rate.
Another type of flow control is that found in U.S. Pat. No. 3,902,521, to Keller et al, wherein a housing of a self-closing valve has a passage lined with a noise-absorbing synthetic-resin and is formed with a valve seat against which a valve body is engageable. The valve body is provided with a throttle element having substantially the same shape as the valve seat. The lining is bonded inside the housing or formed with longitudinal ribs which space it from the interior of the housing so as to form an air-filled insulating space and to form a drain passage for a pilot valve. A compartment of the valve body is connected to the pressurized inlet side of the passage through a small orifice and depressurized by a power belt so as to allow fluid pressure below the belt to pass through the valve seat and thereby allowing flow through the passage.
Another type of flow control is that found in U.S. Pat. No. 4,210,309 to Grenier, wherein a flush valve having a housing, a movable partition sealingly engaging the housing and defining a primary chamber and a control chamber, where there is an inlet port in the primary chamber, an outlet port in the primary chamber and a valve element connected with the partition for closing the outlet port. There is a flow regulating outlet communicating with the outlet port and the auxiliary chamber for discharging fluid vented through the actuator valve. There is also shown a membrane which is operative to affect sealing of chambers from one another.
The problem with many of the prior devices is the number of parts required to provide an effective pressure balanced flow control valve and the requirement that the parts be of a highly polished surface having very close tolerances. This type of construction requires a great deal of maintenance to the parts and eventual replacement; and also exhibits an undesirable sensitivity to any variance in pressure between the outlet and the inlet, which adversely affects the rate of flow of the liquid to be dispensed.